Communication from a stationary or vehicle-based antenna to a skyborne target such as a polar orbiting satellite, a geosynchronous satellite, or an airplane, etc., may require automatic tracking. Various types of reflector antennas are known, and various techniques of generating offset beams, for example, conical scanning, sequential lobing, and single channel monopulse, have proven to be acceptable for automatic tracking of targets.
Typical conical scanning involves the principle of generating an offset beam about the focal axis (tracking axis or boresight) by using a single feed element which is offset and rotated about the focal axis. There are many variations of conical scanning. In general, a conical scan can be produced either electrically (by varying electrical parameters of the antenna) or mechanically (by rotating a component of the antenna).
A Cassegrain reflector antenna includes a parabolic dish for focusing the received signal onto a secondary reflector, or sub-reflector, having a hyperbolic profile, which refocuses the signal onto a feed, located in the center of the parabolic dish. A ring focus reflector antenna is similar to the Cassegrain except that dish is a section of a parabola and the feed is located offset from the center of the dish. The ring focus design is valued for its excellent RF performance in gain, side lobes and efficiency. In the Cassegrain or ring focus configurations, the sub-reflector may be rotated at high speed so as to obtain a mechanical conical nutation within a certain angle of rotation which defines the scan region covered by the antenna beam.
The conical scan can determine errors in the positioning of the antenna, since the amplitude of the received signal will be modulated by the mechanical nutation imparted by the rotating element. When an optimal position has been determined for the antenna, an antenna positioning device, typically an elevation over azimuth actuator, is used to finally position the antenna structure. The primary advantage of conical scanning is its low implementation cost, relative to monopulse.
Single Channel Monopulse (SCM) utilizes a feed, in typically four or five element configurations, and a combining network to generate a reference signal and azimuth and elevation difference signals of a monopulse feed. The azimuth and elevation difference signals are biphase modulated and sequentially coupled to the reference signal. The resultant signal is similar to conical scanning signals because the combined reference and difference signal produces an offset beam relative to the focal axis. However, monopulse tracking feeds are relatively expensive and may not be retrofitted to existing antennas.